This invention relates to a lensed optical connector pin, and more specifically pertains to a lowloss, expanded-beam, fiber optic interconnection having readily cleanable optical faces on the pins effecting an optical connection. The use of rugged fiber couplings effected by SMA-style single-fiber connectors is well known in the art. Such connectors typically employ a bare optic fiber end which is potted with epoxy resin in a precision ferrule. The end of the optic fiber is polished down until a flat, smooth surface is formed with the fiber end flush with the end of the ferrule in which disposed.
Such constructions are subject to well known problems relating to light loss resulting from fiber misalignment in the mating pins. In addition, the small areas defined by the fiber ends which effect light transfer are highly susceptible to dirt contamination and resulting light loss. Such problems arise because the mating regions of the two fibers effecting an interconnection are typically only fifty microns in diameter when multi-mode fibers are employed, and as small as eight microns in diameter when employing single-mode fibers. Also, the well known "pot-and-polish" procedures employed in formation of butt joints by opposing contacts introduce problems of messiness, and require lengthy and costly assembly time, even when a skilled technician performs the contact-forming steps.
The shortcomings of fiber butt joints are well known in the prior art and discussed in the article "Glass Sphere Lenses for Better Coupling" by Norman Nager appearing in the September, 1983, issue of Photonics Spectra, pages 52 through 56. This article discusses the advantages of employing an optical connector utilizing precision spheres rather than a butt joint between fiber ends. In this article, the large light losses and resulting inefficiency of light transfer are noted when the fiber ends defining the butt joint are axially misaligned, have their end faces tilted relative to each other, or are spaced apart so as to leave an intervening gap.
However, when spheres are employed in conjunction with the fiber ends to interface such ends in a fiber optic connection, a beam is developed in one sphere and transferred to a second sphere for passage to an adjoining fiber. The total light insertion loss is typically of the order of 0.70 dB to 1.5 dB, well within accepted light loss levels. This manner of achieving an optical interconnection may be contrasted with the butt coupling method, wherein losses are far more sensitive to fiber end offset in the xy plane. Such xy plane fiber offsets cause optical losses to increase at a rate approximately 5 to 10 times greater than is the case for lensed termination lateral offsets. In addition, fiber misalignments in the Z direction of only 0.001 to 0.005 inch can cause significant light losses in butt joints, whereas lensed terminations can tolerate variations in separation from 0 to 0.1 inch without serious light losses. It is also known that in butt-type connections, light transfer utilizing single mode fibers is especially inefficient as extremely accurate alignment of the small-diameter fiber cores is required.
In a paper entitled "Optical Contacts", by David E. Welsh and Paul Popper, presented at the Proceedings Of The Fourteenth Annual Connector Group in Philadelphia on Nov. 14, 1983, fiber optic contacts employing ball lenses are disclosed for utilization in SMA type connectors as well as in multi-contact MIL-C-38999 connectors. The prior art has recognized the advantages of ball lenses in the formation of light-transmissive connections as compared to butt joints in patents such as Stewart, et al., U.S. Pat. No. 4,304,461. This patent discloses the utilization of fiber wave guides mounted in precision ferrules which are disposed adjacent ball lenses for purposes of effecting light transfer from one fiber end to another. In accordance with the teachings of the Stewart, et al. patent, a spherical lens is fitted into a recess in a precision ferrule which also serves to align the end of a fiber wave guide at the center of the sphere lens. The ball lenses to be employed in accordance with the teachings of this patent have a refractive index between 1.9 and 2.1. If in fact a ball lens has a refractive index within this desired range, the fiber end contacts the ball lens and is epoxied in place. If, however, the refractive index of the ball lens is of the order of 1.78, a suggestion is made that the fiber end should be spaced from the surface of the sphere lens. There is no disclosure whatsoever as to how such spacing can be effected or is possible utilizing the fiber jewels disclosed. In the structure of the Stewart, et al. connector, a ball lens is located in a recess in a fiberguiding precision furrule.
Other prior art references disclosing the use of ball lenses for purposes of effecting a light transmissive connection comprise Khoe, U.S. Pat. No. 4,440,470, in which a transparent coupling material is located between a fiber end and its associated spherical lens. The coupling material has a flat face which is utilized for purposes of preventing reflection at the interface of the lens and fiber. The refractive index of the coupling material is preferably the same as the refractive index of the fiber core.
The utilization of a spherical lens spaced from a fiber end so as to locate the fiber end at the focus of the spherical lens is disclosed in Milan, et al. U.S. Pat. No. 4,468,087.
Nicia, et al., U.S. Pat. No. 4,451,115 discloses a connector employing ball lenses clamped in seats. This patent also discloses shaped fiber holders for adjusting the axial interval of the fiber holders within the housing, and adjusting the fiber ends at the correct distances from the spherical lenses. The Nicia patent requires an adjustment for proper fiber end-ball lens interval. The structure disclosed in Nicia is quite unlike that employed by applicants as it is much more expensive in view of the multiplicity of components. The Nicia structure does not suggest a precise optical pin connector construction which is ready for use merely by performing the steps of assembly, as does applicants' construction.
Masuda, U.S. Pat. No. 4,371,233, also discloses an optical fiber connector employing ball lenses in which the optical fibers are arranged relative to the ball lenses so that the light emitted from one optical fiber is diverged in parallel by one lens. The light is converged again by the other ball lens for focusing at the end surface of the light-receiving optical fiber. The lenses in Masuda are adjustably positionable for desired relation relative to the optical fiber ends. The structure in Masuda suggests nothing of applicants' optical pin.
Khoe, et al., U.S. Pat. No. 4,327,963 is of interest in that it discloses utilization of a transparent coupling medium between a ball lens and entrance surface of a fiber so that reflections of the light passing therebetween is avoided.
Nicia, et al., U.S. Pat. No. 4,265,511 is of interest for its disclosure of a connector employing ball lenses in which the axes of the fibers may be adjusted parallel to the housing tubes in which disposed for minimizing light loss.